Like last year, a Wednesday roll-out is probable because of the Labor Day holiday in the U.S., which falls on Sept. 5. If Apple hosted the event on Tuesday, Sept. 6, employees would have to work on the off day to prepare, and the invited reporters and analysts would have to travel on Labor Day.

Assuming Apple follows 2015's schedule, it will start taking pre-orders on Saturday, Sept. 10, and release the new devices into retail on Friday, Sept. 23.

Last year, pre-orders opened on Saturday, Sept. 12, but the iPhone 6S and 6S Plus did not reach retail until Friday, Sept. 25, a departure for the firm, which had usually left just a week between pre-order and on-sale.

By opening sales this year on Friday, Sept. 16, Apple would be able to book that many more units into its third quarter -- which ends Sept. 30 -- especially important this year because of slowing iPhone sales.